If you sacrifice a non-Human creature you control while you control Mikaeus, the Unhallowed, the creature is put into your graveyard as part of paying Phyrexian Altar’s activation cost (sacrificing a permanent moves it directly to the graveyard and is not destruction) . Because undying is a triggered ability that triggers “When this permanent is put into a graveyard from the battlefield, if it had no +1/+1 counters on it, return it to the battlefield under its owner’s control with a +1/+1 counter on it,” the sacrificed creature will trigger undying if it had no +1/+1 counters when it was put into the graveyard . The undying trigger will go on the stack and (when it resolves) return the creature to the battlefield with a +1/+1 counter, while the Altar’s activated ability still produces its mana as normal .
Step 1
You activate Phyrexian Altar’s ability and pay its cost; as part of that cost you sacrifice a creature, which moves that creature from the battlefield to its owner’s graveyard (sacrificing does not count as destroying the creature) .
Step 2
Because the creature was put into a graveyard from the battlefield and undying is a triggered ability that watches for that event, undying (granted to the creature by Mikaeus) triggers if the creature had no +1/+1 counters on it at that moment .
Step 3
After costs are paid and the activated ability is put on the stack, state-based actions are checked and then any triggered abilities waiting to be put on the stack (including undying) are placed on the stack as described by the rules governing state-based actions and triggers .
Step 4
When the undying trigger resolves, it returns the creature to the battlefield under its owner’s control with a +1/+1 counter on it (if the trigger’s condition was met) .
Step 5
The Phyrexian Altar’s activated ability still resolves and produces its mana; the creature having returned with a +1/+1 counter is now on the battlefield and will not cause a subsequent undying trigger unless it later dies without any +1/+1 counters .
Resolution
You may sacrifice a non-Human creature to Phyrexian Altar; sacrificing is part of paying the Altar’s activation cost and moves the creature to the graveyard without destroying it . If that creature had no +1/+1 counters when it was put into the graveyard, undying (granted by Mikaeus) will trigger and, when that trigger resolves, return the creature to the battlefield with a +1/+1 counter . If the creature had a +1/+1 counter when sacrificed, undying will not trigger and it will remain in the graveyard . If Mikaeus and a non-Human creature you control both go to the graveyard at the same time and that creature had no +1/+1 counters, the undying trigger granted by Mikaeus will still trigger and can return that creature as described in the ruling [2018-12-07] and last-known-information considerations apply .
